Effect of thermal and non thermal processing technologies on the bioactive content of exotic fruits and their products: Review of recent advances

Food Research International 44 (2011) 1875–1887

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Food Research International j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f o o d r e s

Review

Effect of thermal and non thermal processing technologies on the bioactive content of exotic fruits and their products: Review of recent advances A. Rawson a, A. Patras b,⁎, B.K. Tiwari c, F. Noci d, T. Koutchma e, N. Brunton f a

Department of Biochemistry, School of Natural Sciences, National University of Ireland, Galway, Ireland Department of Food Science, University of Guelph, Ontario, Canada Manchester Metropolitan University, Manchester, M146HR, UK d Department of Food Science, University College Dublin, Dublin, Ireland e Guelph Food Research Centre, Agriculture and Agri-Food Canada, Guelph Ontario, Canada f Teagasc, Ashtown Food Research, Dublin 15, Ireland b c

a r t i c l e

i n f o

Article history: Received 14 September 2010 Accepted 28 February 2011 Keywords: Bioactive Phytochemicals Thermal Non thermal Processing

a b s t r a c t Exotic fruits play a vital role in human diet due to the presence of bioactive compounds. Recent research shows the importance of phytochemicals and antioxidants in human health and nutrition. This review summarizes the recent application of both thermal and non-thermal processing technologies on bioactive content of exotic fruits and their products. This review also discusses the impact of processing conditions on the stability of bioactive compounds in exotic fruits and their products. The information provided will be beneficial for further commercialization and exploration of these novel technologies. © 2011 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . Conventional thermal processing . . . . . . . . . 2.1. Blanching . . . . . . . . . . . . . . . . . 2.2. Pasteurization and sterilization . . . . . . . 2.3. Thermal drying . . . . . . . . . . . . . . 3. Novel thermal processing techniques . . . . . . . 4. Non-thermal processing technologies . . . . . . . 4.1. Dense phase carbon dioxide (DPCD) . . . . 4.2. Pulsed electric field (PEF) . . . . . . . . . 4.3. Ozone processing . . . . . . . . . . . . . 4.4. Ultrasound processing . . . . . . . . . . . 4.5. High hydrostatic pressure processing (HHP) 4.6. Radiation processing . . . . . . . . . . . 5. Degradation mechanism . . . . . . . . . . . . . 5.1. Thermal processing . . . . . . . . . . . . 5.2. Non-thermal processing . . . . . . . . . . 6. Strategies to improve retention during thermal and 7. Conclusions and future trends . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . .

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⁎ Corresponding author. E-mail address: [email protected] (A. Patras). 0963-9969/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2011.02.053

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1. Introduction Large assortment of tropical and subtropical fruits, are known as “exotic” fruits. The term exotic includes a number of tropical fruits that are not yet commonly found in global markets but have the potential to do so in view of their appearance, taste, and textural and nutritional quality parameters (Fernandes, Rodrigues, Law, & Mujumdar, 2010). The exotic fruits include but not restricted to mango, guava, passion fruit, star fruit, rose apple, papaya, lime, cupuacu, passiflora, kumquat, pineapple, carambola, feijoa, kiwano, cherimoya, sapodilla, mamey, lychee and longan and are common ingredients which are frequently used in variety of juices, purees and many fruit based deserts (Doyama, Rodrigues, Novelli, Cereda, & Vilegas, 2005; Dube et al., 2004; Kondo, Kittikorn, & Kanlayanarat, 2005). Exotic fruits contribute about 29.7% of total world fruit production. According to FAO, total fruit production in 2008 was 572.4 million tons, out of which 167.55 million tons were exotic fruits (Fig. 1). A large number of epidemiological studies have associated consumption of fruits and their products with decreased risks of degenerative diseases such as cancer and coronary heart disease (Hansen, Purup, & Christensen, 2003). Disease prevention may be due to the presence of health promoting phytochemicals such as carotenoids, flavonoids, phenolic compounds and vitamins (Gardner, White, McPhail, & Duthie, 2000). In this respect, it is of paramount importance to develop processing methods which preserve not only the nutritional and sensorial quality but also the bioactivity of the constituents present in exotic fruits and their products. To date, the application of heat is the most common method for processing food, because of its ability to inactivate microorganisms and spoilage enzymes (PPO, PME, etc.). The use of certain thermal processing techniques (e.g. water immersion, hot air drying) has been favored by the important technological developments experienced over the last few years together with the easier management of the equipment used (Soria & Villamiel, 2010). However, heat processing particularly under severe conditions may induce several chemical and physical changes that impair the organoleptic properties and may reduce the content or bioavailability of some bioactive compounds (Patras, Brunton, Butler, & Downey, 2009; Patras, Brunton, O'Donnell, & Tiwari, 2010; Patras, Brunton, Tiwari, & Butler, 2009; Rawson, Koidis, Rai, Tuohy, Brunton, 2010; Rawson, Hossain, Patras, Tuohy, Brunton, 2011). Therefore, there is a demand for mild processing technologies such as high pressure processing, irradiation, pulsed electric fields, power ultrasound, ozone and oscillating magnetic fields etc. Recent interests in these technologies are not only to obtain high-quality food with “fresh-like” characteristics, but also to provide food with improved functionalities. As a result in the past decade novel non thermal technologies such as high hydrostatic pressure treatment and high intensity pulse electric field treatment have become established. High pressure processed products are now available world wide while pulsed electric field technologies are on the verge of commercialization (Hendrickx & Knorr, 2002; Soliva-Fortuny, Balasa, Knorr, &

1.1 (Cashewapple) 4.2 (Dates)

2.1 (Avacados)

0.7 (Kiwifruit) 0.8 (Figs)

59.2 (Watermelon)

20.5 (Mangoes, mangosteens, guavas)

Martın-Belloso, 2009). In addition to their possible beneficial effects on nutritional and bioactive content many of these novel technologies are more cost-efficient and environment friendly for obtaining premium quality foods which have led to their revival and commercialization (Butz & Tauscher, 2002; Piyasena, Mohareb, & McKellar, 2003; Vikram, Ramesh, & Prapulla, 2005). Many studies have examined the effect of conventional and novel non-thermal processing technologies on generic fruits and their products, However there have been fewer studies pertaining to the effect of novel thermal and non-thermal processing on the bioactive content of exotic fruits and their products. This paper aims to provide a detailed and critical review of the latest applications of conventional, novel thermal and non-thermal processing on bioactive content of exotic fruits. 2. Conventional thermal processing Thermal processing is the most widely used process technology in the food industry which ensures microbiological safety of the products. These methods rely essentially on the generation of heat outside the product to be heated, by combustion of fuels or by an electric resistive heater, and its transference into the product through conduction and convection mechanisms (Pereira & Vicente, 2010). Fruits are highly perishable commodity and are thermally processed to aid their preservation. They are widely processed into juices, smoothies, purees, nectar etc. apart from dehydrated and canned (whole or in pieces). Table 1 provides a summary of the outcomes of studies on the effect of thermal processing on the bioactive content of exotic fruits. The following section discusses some of the thermal processing techniques. 2.1. Blanching Blanching is a processing step which aids in an inactivation of enzymes, thereby maintaining the color, and nutritional aspects of fruit product. Blanching can be carried using many methods including water, steam, vacuum-steam, in-can, and hot-air. Water blanching (e.g. 75–95 °C for 1–10 min) is most commonly employed, as capital and running costs are relatively low. Blanching is generally used as a pretreatment for many processing technique such as prior to freezing and drying. However, since it is a heat treatment it can lead to losses of heat sensitive compounds as shown in Table 1. For example, Pacheco-Palencia et al. (2009) reported the degradation of anthocyanins in fruit puree of two acai fruit species at 80 °C for 1–60 min, where as the non anthocyanin polyphenolic content remained unaffected. Sian and Ishak (1991) reported similar results for anthocyanins in pineapple cubes and, papaya cubes blanched at a range of temperatures and treatment time. While leaching was attributed as the main cause for anthocyanins loss during blanching of pineapple and papaya cubes, however studies have shown that anthocyanins are heat sensitive phytochemicals, which undergo decomposition following thermal treatment (Patras et al., 2009; Patras et al., 2009). Blanching can also affect many other phytochemical groups in exotic fruits. For example, Beyers and Thomas (1979) reported losses of carotenoids in mango fruit blanched at 80 °C for 5 min, and similar results were observed by Sian and Ishak (1991) following pineapple and papaya cubes blanching (Table 1). Many studies have shown that ascorbic acid is a highly thermo labile compound easily degraded on the application of heat. Beyers and Thomas (1979) reported about the degradation of ascorbic acid due to blanching, In addition, Vieira et al. (2000) reported the degradation of ascorbic acid to dehydroascorbic acid, and degradation of dehydroascorbic acid during blanching treatment. 2.2. Pasteurization and sterilization

11.4 (Pineapples)

Fig. 1. Contribution of various exotic fruits to world production (FAO, 2008).

Heat pasteurization of juices and purees is carried out to destroy microorganisms and inactivate pectin methylesterase (PME), however

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Table 1 Effect of thermal processing on bioactive content of exotic fruits. SNO Fruit

Type

Conditions

Parameters affected

References

1

Acai (Euterpe oleracea)

Puree

Processing temp 80 °C for 1, 5, 10, 30, 60 min

Pacheco-Palencia, Duncan, and Talcott (2009)

2.

Acai (Euterpe precatoria) Puree Cupuacu Nectar

3.

Pine apple

Cubes

(↔) Non-anthocyanin polyphenolic content (↓) Anthocyanin (↓) Ascorbic acid (↓) Dehydroascorbic acid (↓) Carotenoid (↓) Anthocyanins

4.

Papaya

Cubes

(↓) Carotenoid (↓) Anthocyanins

Sian and Ishak (1991)

5.

Guava

Fruit slices

(↓) Ascorbic acid

6.

Pink guava

Puree

7.

Jackfruit

Bulb slices

8.

Mango

Fruit

Uddin, Hawlader, Ding, and 2002) Kong, Ismail, Tan, and Rajab (2010) Saxena, Maity, Raju, and Bawa (2010) Kim, Brecht, and Talcott (2007)

9.

Mango

Fruit

10.

Mango

Puree

11. 12.

Mulberry Tamarillo tree

Fruit extract Stored at 70 °C for 10 h Nectar Pasteurized at 80, 90, and 95 °C for 10 min a) Degassed

Thermally treated at 60–99 °C for 0–240 min Blanching at 70, 85 and 100 °C for 2, 4, 6, 8, 10, and 12 min. Drying at 65 °C for 5–6 h Blanching at 70, 85 and 100 °C for 2, 4, 6, 8, 10, and 12 min. Drying at 65 °C for 5–6 h Drying at 30, 40, and 50 °C Drying at 43.79, 50, 65, 80, 86.21 °C for 4–6 h Hot air drying at 50, 60 and 70 °C

(↓) Lycopene, (↓) lipophilic antioxidant capacity (↓) Total carotenoid

Hot water treatment 46.1 °C for 75 min (↔) Gallic acid, (↔) total hydrolysable tannins, (↔) total soluble phenolics, (↔) antioxidant capacity Hot water treatment 46.1 °C for (↓) Gallic acid, (↓) gallotanins, 75–110 min (↓) total soluble phenolics, (↔) antioxidant capacity Treated at 85 and 93 °C for 16 min (↓) Vitamin A, (↓) PPO, (↔) residual POD

b) Not degassed 13.

Pineapple

Juice

Treated at 55–95 °C for 80 min

14.

Cashew apple

Juice

15. 16.

Mango Acerola

Slices Juice/pulp

Heated at 60 and 90 °C for 1, 2, and 4 h Hot air drying at 60 °C for 20 h Industrially processed (pasteurized)

17.

Cashew apple

Juice/pulp

Industrially processed (pasteurized)

18.

Pitanga

Juice/pulp

Industrially processed (pasteurized)

19. 20.

Durian Mango

Juice Fruit

21.

Mango

Fruit

22. 23.

Papaya Litchis

Fruit Fruit

Drying at 88–130 °C Hot water dipping at 46 or 50 °C for 30 or 75 min Blanched at 80 °C for 5 min and canned at 95 °C for 35 min Canned at 95 °C for 45 min Canned at 95 °C for 20 min

(↓) Total anthocyanin, (↓) ascorbic acid

Vieira, Teixeira, and Silva (2000) Sian and Ishak (1991)

Kim, Lounds-Singleton, and Talcott (2009) Vasquez-Caicedo, Schilling, Carle, and Neidhart (2007) Aramwit, Bang, and Srichana (2010) Mertz, Brat, Caris-Veyrat, and Gunata (2010)

(↔) Ascorbic acid, (↓)dehydroascorbic acid, (↔) total carotenoid (↓) Ascorbic acid, (↓)dehydroascorbic acid, (↔) total carotenoid (↑) HMF, (↓) carotenoid

Rattanathanalerk, Chiewchan, and Srichumpoung (2009) (↓) Xanthophylls, (↑) cis isomers of carotenoids Zepka and Mercadante (2009) (↓) Total carotenoid (↓) Total carotenoid Chen, Tai, and Chen (2007) (↓) Quercetin, (↓) kaempferol Hoffmann-Ribani, Huber, and Rodriguez-Amaya (2009) (↓) Quercetin, (↓) kaempferol Hoffmann-Ribani et al. (2009) (↓) Myricetin (↓) Quercetin, (↓) kaempferol Hoffmann-Ribani et al. (2009) (↓) Myricetin (↓) Flavor volatile Chin et al. (2010) (↔) Total carotenoid, (↔) ascorbic acid Djioua et al. (2009) (↓) Ascorbic acid, (↓) carotene

Beyers and Thomas (1979)

(↓) Ascorbic acid, (↓) carotene (↓) Ascorbic acid, (↓) Carotene

Beyers and Thomas (1979) Beyers and Thomas (1979)

Where (↓) refers to decrease in the level. (↑) refers to increase in the level. (↔) refers to neither increase nor decrease in the level.

similar to other thermal practices it can also lead to changes in the bioactive content of the foodstuff. In the case of exotic fruits the burden of evidence leans towards pasteurization leading to a reduction in levels of bioactive compounds possibly due to the severity of these heat processes (Elez-Martinez, Aguiló-Aguayo, & Martín-Belloso, 2006). Pasteurization of mango fruit, puree generally led to a decrease in the levels of vitamin A, phenolics, though total carotenoids and ascorbic acid were reported to be stable though depending on the severity of the process (Djioua et al., 2009; Kim et al., 2007, 2009; Vasquez-Caicedo et al., 2007). Effect of pasteurization has been reported for mulberry fruit extract, durian juice, pineapple juice, and cashew apple juice leading to a decrease in the levels of bioactive components such as total anthocyanin, ascorbic acid, and carotenoid (Aramwit et al., 2010; Chin et al., 2010; Rattanathanalerk et al., 2009; Zepka & Mercadante, 2009). As stated above while most authors have reported that pasteurization leads to a decrease in levels of

various bioactive components there are exceptions. For example, Mertz et al. (2010) reported that the ascorbic acid remained stable during pasteurization at 80, 90, and 95 °C for 10 min in degassed tamarillo nectar, however dehydroascorbic acid degraded following the same process, whereas in the case of non degassed samples of tamarillo nectar there was a considerable decrease in the levels of ascorbic acid and dehydroascorbic acid degraded completely indicating that degassing is an important pre-treatment process and may aid in retention of bioactive compounds during thermal processing. Total carotenoid content was not much affected following similar conditions though there was some degradation in carotenoids, inducing 5,8epoxidation and cis-isomerization of carotenoids in tamarillo nectar. Hoffmann-Ribani et al. (2009) reported the decrease in the levels of quercetin, kaempferol and myricetin in industrially processed acerola, cashew apple and pitanga juice and pulp. In addition, Beyers and Thomas (1979) reported a decrease in the levels of ascorbic acid

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and carotene in canned mango, papaya and litchis. Simple thermal decomposition would appear to be the most likely causes for losses of bioactive compounds following sterilization and this is dependent on their chemical structure with molecules consisting of unsaturated structure being more prone to degradation. Moreover dissolved oxygen in liquid foods can increase the rate of degradation of bioactive compounds and therefore degassing of liquid food prior to pasteurization can help in their higher retention. 2.3. Thermal drying Dehydrated fruits are often used as either a stand alone food product or as constituents of other foods. The drying process assists in the extension of shelf-life as well as reduction in volume of fruits (Prakash, Jha, & Data, 2004). A number of dehydration procedures are employed for fruits and vegetables such as sun drying, cross-flow, fluidized bed, osmotic air drying, oven drying, drum, spray, puff, freeze and microwave drying. Though drying increases the shelf life of the fruits still it may affect the presence and stability of bioactive compounds such as ascorbic acid, polyphenols, and carotenoids due to their sensitivity towards heat. For example Pragati and Dhawan (2003) studied the changes in ascorbic acid during aonla drying. They observed that the retention of ascorbic acid was highest in osmo-air-dried aonla (243.74 mg/100 g), followed by oven-dried (189.10 mg/100 g), direct solar-dried (170.17 mg/100 g), and indirect solar-dried aonla (159.08 mg/100 g) respectively. The loss in ascorbic acid content might be due to oxidation during storage at high ambient temperature (Pragati & Dhawan, 2003). Murthy and Joshi (2007) reported that the retention of ascorbic acid in the samples dried in fluidized bed drying was greater compared to those dried under sun and hot air tray drying. Methakhup, Chiewchan, and Devahastin (2005) reported the loss in ascorbic acid following vacuum or low temperature super heated steam drying (LPSSD) of Indian gooseberry flake at 65 and 75 °C, however LPSSD exhibited generally better retention of ascorbic acid than vacuum drying. Sian and Ishak (1991) reported decreases in levels of carotenoids and anthocyanins in pineapple and papaya cubes after hot air drying at 65 °C for 5–6 h. They also noted that carotenoids were more stable than anthocyanins to drying. Similar results were reported by Uddin et al. (2002), Kong et al. (2010), and Saxena et al. (2010) following drying of guava fruit slices, sapota fruit, pink guava puree and jackfruit bulb slices respectively for ascorbic acid, total phenol, lycopene, lipophilic antioxidant activity and total carotenoids. In contrast, Saxena et al. (2010) reported gallic acid, and total hydrolysable tannins to be stable following hot air drying of jackfruit bulb slices. 3. Novel thermal processing techniques Novel thermal processing techniques include but are not restricted to ohmic heating, dielectric heating/microwave heating/radio frequency heating, and they may be used as such or in combination to other processing methods. The basic idea residing behind these novel thermal processing methods is the mode of heat transfer in food, for example ohmic heating also known as electrical resistance heating utilizes the resistance offered by the food material in the flow of electrical current through it leading to generation of heat. It is a rapid heating method and has been suggested to be more uniform than other electroheating techniques (Morrissey & Almonacid, 2005). Whereas microwave technology uses electromagnetic waves that pass through food material and cause its molecules to oscillate, generating heat. Fewer studies have been conducted on the use of novel thermal processing methods on exotic fruits, and Yildiz, Bozkurt, and Icier (2009) demonstrated that ohmic heating did not cause any different effect in other quality indices and total phenolic contents of pomegranate juice than the conventional heating.

Given the fact that thermal processing in general leads to a loss of bioactive compounds if processors desire to produce exotic fruit products it may be time for them to re-evaluate existing thermal process treatments with view to using techniques that maximize the bioactive compound content while also resulting a stable product. This has resulted in research efforts directed at novel non thermal processes that can ensure product safety yet maintain the desired bioactive compound retention (Table 2). These technologies are preservation treatments that are effective at ambient or sub-lethal temperatures, thereby minimizing negative thermal effects on food nutritional and quality parameters. 4. Non-thermal processing technologies 4.1. Dense phase carbon dioxide (DPCD) Dense phase carbon dioxide processing (DPCD or DP-CO2), is a collective term for liquid CO2 and supercritical CO2 or high pressurized carbon dioxide (HPCD), it is a non-thermal alternative to heat pasteurization for liquid foods, and it is attracting much interest in the food industry world-wide (Del Pozo-Insfran, Balaban, & Talcott, 2006). Carbon dioxide, a natural constituent of many foods, is a nontoxic, nonflammable, and inexpensive gas. It has Generally Recognized as Safe status. Dense phase carbon dioxide or supercritical CO2 denotes phases of matter that remain fluid, yet are dense with respect to gaseous CO2. Moreover, in the supercritical state, CO2 has low viscosity (3–7 × 10− 5 Pa s) and zero surface tension, so it can quickly penetrate complex structures and porous materials (Zhang et al., 2006). Fraser (1951) was the first to show that DPCD can inactivate bacterial cells. One of the first food applications of DPCD was treatment of whole fruits such as strawberry, honeydew melon, and cucumber to inhibit mold growth (Damar & Balaban, 2005). However, DPCD may cause severe tissue damage in some fruits even at low pressures (Haas et al., 1989). Dense phase CO2 processing is a continuous, non thermal processing system for liquid foods that utilizes pressure (b90 MPa) in combination with carbon dioxide (CO2) to destroy microorganisms as a means of food preservation (Del Pozo-Insfran et al., 2006). Recently Garcia-Gonzalez et al. (2007) reviewed the effects of high pressure CO2 on microbial inactivation. Application of DPCD has been reported for various fruit based products (juices) such as apple cider (Gunes, Blum, & Hotchkiss, 2006); orange juice (Balaban, 2003); grape juice (Gunes, Blum, & Hotchkiss, 2005) and mandarin juice (Yagiz, Lim, & Balaban, 2005). There are a limited number of studies available in the literature regarding the effect of DPCD on bioactivity of exotic fruits. Chen et al. (2010) investigated the effects of DPCD treatment of 8, 15, 22, 30 and 35 MPa for 5, 15, 30, 45, 60 min at 35, 45, 55, 65 °C on vitamin C in Hami melon juice during storage at 4 °C for 4-weeks. The authors found that vitamin C concentration decreased following DPCD processing, but percentage loss was lower than of the untreated samples. This could be because CO2 can lower pH when dissolved in the aqueous part of foods but ascorbic acid has higher stability at low pH and oxidizes easily when oxygen is present in the environment. They concluded that specific investigations might be required to determine the parameters required to optimize the quality and bioactive content of melon juice. The same authors carried out a separate study comparing thermal (90 °C for 60 s) and DPCD processing (55 °C, 60 min, and 35 MPa). The authors concluded that conventional HTST pasteurization inactivated microorganisms in the melon juice, but caused significant losses in ascorbic acid content. Plaza, Ramirez-Rodrigues, Balaban and Balaban (2010) investigated the effects of DPCD (30.6 MPa, 8% CO2 and 6.8 min, 35 °C) treatments on ascorbic acid content of guava puree and compared it with thermal treatments (90 °C for 60 s). Satisfactory retention of ascorbic (p b 0.05) in DPCD treated samples was observed by the authors. The effect of

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Table 2 Array of non-thermal technologies for processing exotic fruits and their products. SNO Technology

Description of technology

Fruit

Product type

References

1

High hydrostatic pressure processing

Liquid/solid foods, with/without packaging (100–800 MPa, below 0 °C to N100 °C, from a few seconds to over 20 min)

Juice Puree Juice Juice puree Smoothie

Ferrari, Maresca, and Ciccarone (2010) Butz et al. (2003) Lavinas, Miguel, Lopes, and Mesquita (2008) Wolbang, Fitos, and Treeby (2008) Yen and Lin (1996) Keenan et al. (2010)

2

High-intensity pulsed electric fields (HIPEF)

High voltage pulses to foods between two electrodes (b 1 s; 20–80 kV/cm; exponentially decaying, square wave, bipolar, or oscillatory pulses at ambient, sub-ambient, or above ambient temperature)

Pomegranate Peach Cashew apple Melon guava Banana (strawberry, orange blend) Orange, kiwi, pineapple Orange, kiwi, pineapple Cherry Watermelon

Fruit juice– soymilk Fruit juice– soymilk Juice Juice

Morales-de la Peña, Salvia-Trujillo, Rojas-Graü, and Martín-Belloso (2010a,b) Morales-de la Peña et al. (2010a,b)

Strawberry

Juice

Hami melon Peach

Juice Buffer

Altuntas, Evrendilek, Sangun, and Zhang (2010) Oms-Oliu, Odriozola-Serrano, Soliva-Fortuny, and Martín-Belloso (2009) Odriozola-Serrano, Soliva-Fortuny, and Martín-Belloso (2009) Chen et al. (2009) Zhou, Zhang, Hu, Liao, and He (2009)

Kiwi

Cubes

Barboni, Cannac, and Chiaramonti (2010)

Watermelon

Juice

Rawson, Tiwari et al. (2010)

Watermelon Pomegranate Mangoes

Cubes Arils Cubes

Fonseca and Rushing (2006) Lopez-Rubira, Conesa, Allende, and Artés (2005) González-Aguilar, Villegas-Ochoa, Mart nez-Téllez, Gardea, and Ayala-Zavala (2007)

3

4

5 6

Dense-phase carbon dioxide

Carbon dioxide, a natural constituent of many foods, in the supercritical state, CO2 has low viscosity (3–7 × 10− 5 Pa s) and zero surface tension, so it can quickly penetrate complex structures and porous materials Ozone processing Ozone is a triatomic allotrope of oxygen and is characterized by a high oxidation potential that conveys bactericidal and viricidal properties Ultrasound Energy generated by sound waves of 20,000 Hz or more Ultraviolet (UV-C) light UV radiant exposure, at least 400 J/m2, intense and short-duration pulses of broad spectrum (ultraviolet to the near infrared region)

DPCD on ascorbic acid is interesting as most thermal and even many non-thermal methods lead to significant losses. DPCD also appear to prevent losses of other potential bioactive compounds such as β-carotene. The study conducted by Chen et al. (2010) showed better retention of β-carotene in DPCD (55 °C, 60 min, and 35 MPa) treated melon juice compared to conventional HTST pasteurization. Significant losses (p b 0.05) (57.87%) in β-carotene content was observed in heat pasteurized samples. It should be noted that exact mechanism for β- carotene stability is difficult to establish. Some authors have also focused on the effect of DPCD on the total antioxidant properties of exotic fruits. Plaza et al. (2010) investigated the effects of DPCD (30.6 MPa, 8% CO2 and 6.8 min, 35 °C) treatments on total antioxidant activity and phenolic content of guava puree and compared it with thermal treatments (90 °C for 60 s). In their study, total antioxidant activity (DPPH) in organic fractions was not changed (p b 0.05) by DPCD treatments. Similar results were reported for phenolic content. Literature reveals that most of the reported applications of DPCD are limited to vitamin C, carotenoid and total antioxidant activity and there is also lack of information on the behavior of individual polyphenols (anthocyanins, phenolic acids etc.). 4.2. Pulsed electric field (PEF) Pulsed electric field is a technology that has been extensively investigated in recent years for its applications in food processing. By the mechanism of electropermeabilization, pulsed electrical fields have proved a valid technology for the production of safe beverage products and shown a positive influence in the texture of solid plant foods, leading to enhanced yields of extraction of metabolites, as well as increased juice yields. One of the principal differences in the use of PEF consists of the intensity of the field used, as higher intensity fields (15–40 kV/cm, 5–100 pulses, 40 to 700 μs 1.1 to 100 Hz) (Zulueta, Esteve, & Frígola, 2010) are more effective towards microbial inactiva-

tion, while low and medium intensity fields (0.6–2.6 V/cm, 5–100 pulses, short treatment time within 10− 4–10− 2 s; 1 Hz) have been successfully used for enhancing mass transfer in solid foods (Corrales, Toepfl, Butz, Knorr, & Tauscher, 2008).With increasing interest in availability of bioactive compounds from fresh and processed foods, the effect of this processing technology in relation to the different bioactive compounds present in fruit has been reviewed comprehensively by Soliva-Fortuny et al. (2009). While the effects on bioactive compounds in products such as orange juice and grape juice have been extensively investigated, there is limited amount of literature available in relation to bioactive compounds present in exotic fruit and fruit juices. However there are some examples of application of this technology to exotic fruit based beverages. Morales-de la Peña et al. (2010a,b) investigated the effect of PEF on vitamin C in juice from exotic fruit based drinks immediately after treatment and concluded that levels were not different from the thermally processed juice in an orange/kiwi/pineapple and soymilk based drink. However, the beneficial effect of the PEF treatment was noticeable over a storage period of 31 days, as an 800 μs treatment at 35 kV/cm showed significantly greater retention than both a 1400 μs treatment and a thermal treatment. These results showed that the shorter the HIPEF treatment time, the higher the vitamin C retention, as previously found in other studies focused on individual fruit juices treated by HIPEF. In general, longer exposure PEF treatment times may induce reduction in the product retention of vitamin C due to product heating. Longer exposure time may also generate free radicals which may speed up vitamin C degradation. In watermelon juice, a product with a low initial concentration of vitamin C, a treatment with 25 kV/cm for a relatively short time (50 μs, 50 Hz) produced a relative vitamin C content of 99.9%, compared to increasingly lower relative vitamin C values as both the treatment time and the electric field strength were increased (45.8% at 35 kV/cm and 2050 μs) (Oms-Oliu et al., 2009), but this treatment might be appropriate for product safety. In a separate study conducted by (Altuntas et al., 2010), demonstrated total anthocyanin stability in content of sour cherry

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juice was well retained when treated by PEF treatments (17–30 kV/cm for 131 μs). In cases where PEF has induced a loss of anthocyanins this is probably due to a direct impact of the treatment on these compounds and to the partial inactivation of enzymes (β-glucosidase, peroxidase (POD) and polyphenoloxidase (PPO)) that is induced by PEF processing. Aguiló-Aguayo et al. (2008) reported an increase in the activity of β-glucosidase in strawberry juice, which can explain the corresponding degradation of anthocyanin following PEF treatment at 35 kV/cm for 1000 μs at 50 Hz. The total phenolic content of a blend of orange, kiwi, pineapple juice and soymilk was not affected by PEF treatments conducted at 35 kV/cm, 4 μs bipolar pulses at a frequency of 200 Hz for a total treatment time of 800 μs and 1400 μs. No effect of PEF was detected on phenolic compounds. However, significant reductions were observed for vitamin C concentration which were reflected in a decrease in the antioxidant activity of the product. Under the tested processing conditions the PEF treatment caused a reduction in the vitamin C and antioxidant capacity which decline over time compared to conventional thermal treatment (Morales-de la Peña et al., 2010a, 2010b). The effect of PEF processing on the bioactive compounds in watermelon juice was extensively studied by Oms-Oliu et al. (2009). While severe PEF strength proved to increase the rate of vitamin C loss in the juice, the lycopene retention in HIPEF-processed watermelon juice ranged from 87.6% to 121.2% over the range of processing parameters (field strength 25–35 kV/cm, frequency 50–250 Hz, pulse width 1–7 μs and treatment time 50–2050 μs). Enhancement of lycopene content may be due to PEF-induced cell permeabilization and release of intracellular pigments (lycopene) from watermelon. It is envisaged that such an increase at these electric field intensities could have been stress induction in watermelon cells and subsequent production of lycopene as secondary metabolite stimulating metabolic activity and accumulating secondary metabolite (Guderjan, Toepfl, Angersbach, & Knorr, 2005).These results were similar to those observed by Odriozola-Serrano et al. (2009) in strawberry juice processed by PEF at a constant field strength (35 kV/cm) and treatment time (1000 μs) where the presence of health-related compounds (vitamin C, anthocyanins and antioxidant capacity) was found to be at its maximum at a treatment frequency of 232 Hz and a pulse width of 1 μs. Under all experimental conditions the relative retention of anthocyanins ranged between 87 and 102%, which was greater than that reported. The antioxidant capacity of a mix of kiwi, orange and pineapple juice and soy milk was not affected by the treatment with PEF, regardless of the total treatment time (800 or 1400 μs) and was found not to be different from thermally processed product (90 °C, 60 s). Moreover, the antioxidant capacity of this product during storage decreased to a greater degree in thermally treated samples than in PEF-treated samples after a storage period of 60 days (Morales-de la Peña et al., 2010a, 2010b). The main causes of degradation in antioxidants during thermal processing are oxidation and isomerization (Shi & Le Maguer, 2000). The total antioxidant capacity of watermelon juice was affected by the treatment conditions in a study by Oms-Oliu et al. (2009), processing with 35 kV/cm field strength at 250 Hz for 2050 μs did not seem to affect the antioxidant capacity in the juice when treated with a 7 μs pulse width, though was significantly reduced when the pulse width used was 1 μs and the frequency reduced to 50 Hz. Distinct from the application of PEF for preservation purposes, electrical field strengths aiming at increasing extraction yields of metabolites from plant foods are generally in the range of 1–10 kV/cm (Ade-Omowaye, Angersbach, Eshtiaghi, & Knorr, 2001; Corrales et al., 2008; Lopez, Puertolas, Condon, Raso, & Alvarez, 2009). At these field strengths microbial inactivation is negligible for most species and the extent of cell poration tends to be reversible. However in general lower field strength has proved to successfully increase the rate of expression of juices and, subsequently, the metabolites, from solid food matrices.

The application of this technology as a treatment to enhance yield extraction has been reported in relation to several plant foods (e.g. apple, sugar beet, grapes, carrot) (Corrales et al., 2008; El-Belghiti, Rabhi, & Vorobiev, 2007; El-Belghiti & Vorobiev, 2004; Lopez et al., 2009). Successful application of PEF technology has been reported by Ade-Omowaye et al. (2001) as a pre-processing step in coconut processing, with an increase in milk yield, though no data on the bioactive compounds in the coconut milk following processing was reported. However, to date there are no reports focusing on the application of similar methods of extraction of juice and/or on the effect of such processing technology on bioactive metabolites from exotic fruit sources. Although the existing data available on other plant foods may provide a solid base for studies on bioactive compounds, it is the opinion of these authors that this field may require further investigation. 4.3. Ozone processing The interest in ozone as a preservation technology is based on its high biocidal efficacy and wide antimicrobial spectrum. Within the food industry, ozone has been used routinely for washing and storage of fruits and vegetables by gaseous treatment. With the recent FDA approval of ozone as a direct additive to food, the potential of ozonation in liquid food applications has begun to be exploited (Cullen, Tiwari, O'Donnell, & Muthukumarappan, 2009). Ozone as an antimicrobial agent has numerous potential applications in the food industry because of its advantages over traditional antimicrobial agents such as chlorine, potassium sorbates, etc. The use of ozone application for the disinfection or storage of various exotic fruits or their products including kiwi fruits has been reported (Akbas & Ozdemir, 2008; Barboni et al., 2010; Graham & Tyman, 2002; Hur et al., 2005; Meyvaci, Sen, & Aksoy, 2010; Oztekin, Zorlugenc, & Zorlugenc, 2006; Whangchai, Saengnil, & Uthaibutra, 2006; Zorlugenc, Zorlugenc, Oztekin, & Evliya, 2008). However, most of the reported studies are limited to microbiological analysis of exotic fruits. Typically, ozone at concentrations of 0.15–5.0 ppm has been shown to inhibit the growth of spoilage bacteria as well as yeasts (Jay, Loessner, & Golden, 2005). Kiwi fruit is a rich source of vitamin C and contains more ascorbic acid than citrus fruits (Nishiyama et al., 2004). Barboni et al. (2010) compared the effect of ozone rich storage and air storage over a period of 7 months on the vitamin C content of kiwi fruit. Gaseous ozone concentration was 4 mg/h in the chamber at a temperature of 0 °C and a humidity of 90–95%. The authors did not observe any significant change in ascorbic acid content of kiwi fruit over a 7 month storage period at an ozone concentration of 4 mg/h in the chamber (2 m3) and a storage temperature of 0 °C. Reports on the effect of ozone on other bioactive compounds of exotic fruits are limited. However, ozone would be expected to cause the loss of antioxidant bioactive compounds, because of its strong oxidizing activity. Contradictory reports were found in the literature regarding ascorbic acid. Perez, Sanz, Rios, Olias, and Olias (1999) studied the effect of ozone treatment on the postharvest quality of strawberry stored at 2 °C in an atmosphere containing ozone (0.35 ppm). Absolute increases in ascorbic acid levels in strawberries were reported in response to ozone exposure. Ozone treatments were also reported to have minor effects on anthocyanin contents in strawberries (Perez et al., 1999) and blackberries (Barth et al., 1995). Alothman, Bhat, and Karim (2009a,b) investigated the effect of ozone treatment on total phenol, flavonoid, and vitamin C content of fresh-cut honey pineapple, banana ‘pisang mas’, and guava. Fruits were exposed to ozone at a flow rate of 8 ± 0.2 ml/s for 0, 10, 20, and 30 min. Total phenol and flavonoid contents of pineapple and banana increased significantly when exposed to ozone for up to 20 min, with a concomitant increase in FRAP and DPPH values. The opposite was observed for guava. Ozone treatment significantly decreased the vitamin C content of all three

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fruits. The main factor that could have contributed to the degradation of ascorbic acid in the three ozone-treated fruit types is the activation of ascorbate oxidase. This enzyme is activated under stress conditions, such as chemical exposure. Ascorbate oxidase has been reported to promote the degradation of ascorbic acid to dehydroascorbic acid (Lee & Kader, 2000). In contrast, this study showed promising results for enhancing antioxidant capacity of some fresh fruits by ozone treatment although the positive effect is compromised by a reduction in vitamin C content. Applying ozone at doses that are large enough for effective decontamination may change the sensory qualities of food. Ozone is not universally beneficial and in some cases may promote oxidative spoilage in foods. Surface oxidation, discoloration or development of undesirable odors may occur in substrates from excessive use of ozone (Khadre et al., 2001). Studies show that the effects of ozone on physiology and quality of fruits vary according to chemical composition of food, ozone dose, and application type and time (Cullen et al., 2010; Karaca & Velioglu, 2007; Whangchai et al., 2006). 4.4. Ultrasound processing The use of ultrasound within the food industry has been a subject of research and development for many years. In last decade power ultrasound has emerged as an alternative processing option to conventional thermal approaches for pasteurization and sterilization of food products. Ultrasound processing on its own or in combination with heat and/or pressure is an effective processing tool for microbial inactivation and phytochemical retention. Advantages of ultrasound include reduced processing time, higher throughput, and lower energy consumption (Zenker, Heinz, & Knorr, 2003). It is certainly capable of achieving a desired 5 log for food borne pathogens in fruit juices (Salleh-Mack & Roberts, 2007) but there is some evidence that it could negatively modify some food properties including flavor, color, or nutritional value. Ultrasound treatment of fruit juices is reported to have a minimal effect on the ascorbic acid content during processing and results in improved stability during storage when compared to thermal treatment. This positive effect of ultrasound compared with heating is assumed to be due to the effective removal of occluded oxygen from the juice (Knorr, Zenker, Heinz, & Lee, 2004) as this is a critical parameter influencing the retention of ascorbic acid. With regard to exotic fruits Cheng, Soh, Liew, and Teh (2007) reported a significant increase in the ascorbic acid content of guava juice during sonication from 110 ± 0.5 (fresh) to 119 ± 0.8 (sonication) and to 125 ± 1.1 (combined sonication and carbonation) mg/100 ml which could be due to cavitations effects caused by carbonation and sonication, respectively. The authors also observed that during carbonation, sample temperature decreased substantially which could have disfavored ascorbic acid degradation. Rawson, Tiwari et al. (2010) investigated the effect of thermosonication on the bioactive compounds of freshly squeezed watermelon juice. They observed a higher retention of ascorbic acid and lycopene at low amplitude level and temperature. They also observed a slight increase in lycopene at low amplitude level. Ultrasound processing is also reported to enhance extraction yield of bioactive compounds by about 6 and 35% (Vilkhu et al., 2008) though depending on the processing conditions. Rawson, Tiwari et al. (2010) reported that sonication temperature played a significant role in preservation of bioactive compounds. They observed a decrease in the phenolic content of sonicated watermelon juice when the temperature was increased from 25 to 45 °C. Temperature effect was more pronounced at higher processing times (e.g. 10 min). 4.5. High hydrostatic pressure processing (HHP) High hydrostatic pressure processing (HHP) is an established non thermal food processing and preservation technique with reduced

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effects on nutritional and quality parameters compared to conventional thermal processing. HHP is derived from material science in which products are treated above 100 MPa. HPP has been extensively reviewed (Rastogi, Raghavarao, Balasubramaniam, Niranjan, & Knorr, 2007). The great advantage of HHP treatment is that pressure at a given position and time is the same in all directions, transmitted uniformly, immediately through the pressure transferring medium and independent of geometry (Oey, Plancken, Loey, & Hendrickx, 2008). Literature indicates that high pressure processing (HPP) preserves the nutritional value of HPP processed food and food products. HPP treatment at ambient temperature is reported to have minimal effect on the bioactive content of various fruits and vegetables (Oey et al., 2008) but fewer studies have been conducted on exotic fruits. Yen and Lin (1996) investigated the effects of high pressures and thermal pasteurization on ascorbic acid (AA) content of guava puree during storage at 4 °C. After treatment at a pressure of 600 MPa and 25 °C for 15 min, the product exhibited no change in AA content as compared with fresh samples. The authors concluded that guava puree treated at 600 MPa and 25 °C for 15 min retained good quality similar to the freshly extracted puree after storage at 4 °C for 40 days. Evolution of AA content in pressure treated food products during storage has been followed. It is suggested that further AA degradation after HHP processing could take place during storage and it could be eliminated by lowering storage temperature. Furthermore, it has been reported that different HHP combinations had different influences on the stability of vitamin C in guava puree during storage (Yen & Lin, 1996). The ascorbic acid content of untreated and pressurized (400 MPa/room temperature/15 min) guava puree started to decline after 10 and 20 days, respectively, whereas it remained constant in thermal (88–90 °C/24 s) and in higher pressure (600 MPa/room temperature/15 min) treated guava puree during 30 and 40 days, respectively. The latter could be caused by the inactivation of endogenous pro-oxidative enzyme during treatment at high pressure level. At elevated temperatures, pressure treatment could degrade vitamin C to a large extent for long treatment time, e.g., pressurization up to 600 MPa at 75 °C for 40 min resulting in 70% and 50% losses of vitamin C, respectively, in pineapple and grapefruit juices (Taoukis et al., 1998). At constant pressure, increasing temperature enhanced the vitamin C degradation, for example loss 20–25% at 40 °C; 45–50% at 60 °C and 60–70% at 75 °C at 600 MPa for 40 min in pineapple juice (Taoukis et al., 1998). Ferrari et al. (2010) investigated the effects of high pressures (400–600 MPa) at 25, 45, 50 °C for 5 or 10 min on phytochemical (anthocyanins, polyphenols) content of pomegranate juice. Their experimental results indicated that the content of anthocyanins was influenced mainly by pressure and temperature level. At room temperature, the concentration of these molecules decreases with the intensity of the treatment in terms of pressure level and processing time. Therefore, the higher pressure levels or longer processing times caused both a decrease of the anthocyanin content. The authors indicated that high pressure treatments modified the mechanism of anthocyanin degradation by affecting the molecules involved in the kinetics of reaction, such as enzymes. The residual activity of the enzymes along with a small concentration of dissolved oxygen could cause the degradation of the anthocyanins during the storage of the processed juice, as widely supported by studies reported in current scientific literature (Suthanthangjai, Kajda, & Zabetakis, 2005; Zabetakis, Leclerc, & Kajda, 2000). Distinct from the application of HPP for preservation purposes, high pressure treatments have extensively used to extract secondary plant metabolites from fruits and vegetables. For example, De Ancos, Gonzalez, and Pilar Cano (2000) successfully employed HHP (50–400 MPa, 25 °C/15 min) processing to extract carotene from persimmon fruit purees. Different pressure levels at constant temperature gave different release of

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various carotenes depending on their chemical properties and chromoplast location. The extraction of bioactive compounds can be described as a mass transport phenomenon where solids contained in plant structures migrate into the solvent, up to their equilibrium concentration. Mass transport phenomenon can be increased by heating, changes in concentration gradients, and under the influence of new technologies such as ultrasonics, high pressure, and pulsed electric field (Corrales et al., 2008). The application of high hydrostatic pressure was first used as an extraction technique in plant systems by Zhang, Junjie, and Changzhen (2004). The use of high pressure enhances mass transfer rates, which increases cell permeability as well as secondary metabolite diffusion (Dornenburg & Knoor, 1993). Prasad et al. (2010) extracted longan fruit pericarp with 50% ethanol employing high pressure (500 MPa) and conventional extraction methods. Their study demonstrated that high pressure extraction (HPEL) showed excellent antioxidant and anticancer activities and was higher than conventional extraction (CEL). Three phenolic compounds, namely gallic acid, corilagin, and ellagic acid, were identified and quantified by external standard methods. Compared with CEL, HPEL exhibited higher extraction effectiveness in terms of higher extraction yield, higher phenolic content, and higher antioxidant and anticancer activity with shorter extraction time. Increased extraction yields caused by high pressure are probably due to its aptitude to deprotonate charged groups and to disrupt salt bridges and hydrophobic bonds in cellular membranes, which may lead to a higher permeability (Dornenburg & Knoor, 1993). It is obvious that HPP treatment influences the phytochemical stability and the extraction yield of bioactive compounds. As a consequence, changes in antioxidant activity could also occur during HPP treatment. The authors also suggested that high-pressure treatment may provide a better way of utilizing longan fruit pericarp as a readily accessible source of the natural anticancer and antioxidant in food and pharmaceutical industry. 4.6. Radiation processing Irradiation treatment generally involves the exposure of food products (raw or processed) to ionizing or non-ionizing radiation for the purpose of food preservation. The ionizing radiation source could be high-energy electrons, X-rays (machine generated), or gamma rays (from cobalt-60 or cesium-137), while the non-ionizing radiation is electromagnetic radiation that does not carry enough energy/quanta to ionize atoms or molecules, represented mainly by ultraviolet rays (UV-A, UV-B, and UV-C), visible light, microwaves, and infrared. Irradiation of food products causes minimal modification in the flavor, color, nutrients, taste, and other quality attributes of food (Alothman et al., 2009b). However, the levels of modification (in flavor, color nutrients, taste etc.) might vary depending on the basic raw material used, irradiation dose delivered, and on the type of radiation source employed (gamma, X-ray, UV, electron beam) (Bhat & Sridhar, 2008; Bhat, Sridhar, & Yokotani, 2007). Depending upon the radiation dose, foods may be pasteurized to reduce or eliminate food-borne pathogens. Inactivation of microorganisms by irradiation is primarily due to DNA damage, which destroys the reproductive capabilities and other functions of the cell (DeRuiter & Dwyer, 2002). Application of gamma radiation on pomegranate juice (Alighourchi, Barzegar, & Abbasi, 2008), and UV radiation on orange, guava-andpineapple juice (Keyser, Muller, Cilliers, Nel, & Gouws, 2008) has been reported for the inactivation of microorganisms. Irradiation induces negligible or subtle losses of bioactive compounds as it does not substantially raise the temperature of food during processing (Wood & Bruhn, 2000). However, Alighourchi et al. (2008) reported a significant reduction in the total and individual anthocyanin content in pomegranate juice after irradiation at higher doses (3.5–10 kGy). Irradiation effects on anthocyanin pigments depend upon the nature of anthocyanin for

example; diglycosides are relatively stable towards irradiation dose compared to monoglycosides. Reyes and Cisneros-Zevallos (2007) also investigated the effect of irradiation (1–3.1 kGy) on mango. The authors did not find a significant affect of irradiation dose on the total phenolic content while there was a significant increase in flavonols after an 18 day storage period for the irradiated fruits (at 3.1 kGy). In contrast, ascorbate content of the fruits decreased when the dose exceeded 1.5 kGy. No major changes in the carotenoids content were recorded. In general, the decrease in antioxidant compounds is attributed to the formation of radiation-induced degradation products or the formation of free radicals (Sajilata & Singhal, 2006; Wong & Kitts, 2001). Alothman et al. (2009a,b) investigated the effect of ultraviolet (UV-C) treatment on total phenol, flavonoid, and vitamin C content of fresh-cut honey pineapple, banana “pisang mas” and guava. On average, the samples received a UV radiation dose of 2.158 J/m2. In their study, total phenol and flavonoid contents of guava and banana increased significantly with the increase in treatment time (p b 0.05). In pineapple, the increase in total phenol content was not significant (p N 0.05), but the flavonoid content increased significantly after 10 min of treatment. In contrast UV-C treatment decreased the vitamin C content of all three fruits. A separate study conducted by Lopez-Rubira et al. (2005) demonstrated insignificant changes in anthocyanins and antioxidant activity of pomegranate arils after exposure to UV-C (0.56–13.62 kJ/m2). Fresh-cut mangoes UV-C irradiated for 0, 10, 20, and 30 min, showed an increase in phenolic compounds and flavonoid contents with the increase in treatment time, while both β-carotene and ascorbic acid decreased (González-Aguilar et al., 2007). Fan, Toivonen, Rajkowski, and Sokorai (2003) reported that the free radicals generated during irradiation might act as stress signals and may trigger stress responses in lettuce, resulting in an increased antioxidant synthesis. Irradiation of plant tissues with UV has been shown to have positive interactions, indicating an increase in the enzymes responsible for flavonoid biosynthesis, affecting plant phenolic metabolites apart from induction of abiotic stress. UV-A has been reported to induce anthocyanin biosynthesis in fruits encompassing cherries (Kataoka, Beppu, Sugiyama, & Taira, 1996). It is quite evident that, apart from the application of UV for microbial safety at industrial levels, this novel technology has some potential in enhancement of health promoting compounds with some exceptions. 5. Degradation mechanism 5.1. Thermal processing Degradation of bioactive compounds in exotic fruits is rather complex and entirely dependent on the type of processing. Better understanding of the oxidative degradation of bioactive compounds is needed not only in avoiding its loss during processing and storage of foods, but also because of possible implications to human health. However, systematic studies to demonstrate the occurrence of these reactions are lacking. Insights into the mechanism of carotenoid oxidation can be derived from food model systems, which are more easily controlled than foods and the formation of initial, intermediate, and final products can be more easily monitored. Vitamins are among the most sensitive food components in exotic fruits to be affected by heat sterilization. Loss of vitamin C tends to follow consecutive first-order reactions; i.e., a rapid oxygen-dependent reaction that proceeds until oxygen is depleted, followed by anaerobic degradation. Vitamin C degradation mechanism is specific of a particular system, as it depends on several factors (processing kind) (Tannenbaum, 1976), following either an aerobic or anaerobic pathway. In the aerobic pathway oxidation can follow a catalyzed pathway or an uncatalyzed pathway. Both pathways have common intermediate products that cannot be distinguished by chemical analysis and both lead to dehydroascorbic acid (DHAA) which by further degradation forms 2, 3-diketogulonic acid (DKGA). In the anaerobic pathway ascorbic acid undergoes ketonization

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to form the intermediate keto-tautomer (keto-ascorbic acid) which is in equilibrium with its anion (keto-monoanion ascorbic acid) which by further delactonization forms DKGA (Tannenbaum, 1976). Carotenes and xanthophylls are also abundantly present in exotic fruits. These bioactive compounds are also sensitive to oxygen, heat, and light and their stability may be influenced by thermal pasteurization. Additionally, exposure to heat or oxygen may induce carotenoid isomerization, with cis-isomer configurations reported to have greater antioxidant capacity than their parent compounds. Zepka and Mercadante (2009) studied the influence of organic acid and heating treatments on carotenoid degradation on a simulated cashew apple juice. In their study, the simulated cashew apple juice was heated for 1, 2, and 4 h at 60 °C and 90 °C in a temperature controlled water bath. Taking into account the structures of the thermal degradation products formed in both β-cryptoxanthin heated aqueous systems, isomerization from trans to cis configurations, epoxidation and cleavage were the main degradation reactions observed in the present study (Fig. 2). Moreover, degradation to non-colored compounds was also observed since the amounts of β-cryptoxanthin degraded were not compensated by the amounts of products formed. The same authors also studied the effect of heating on carotenoid degradation in a simulated cashew apple juice. In general, as expected, the levels of all-trans carotenoids decreased with a concomitant increase in the amounts of cis isomers and oxidation products, as time and heating temperature increased. Moreover, breakdown non-colored compounds were also formed since the increased levels of cis isomers and oxidation compounds did not compensate for the losses of total carotenoid contents. Heating at both temperatures also caused a significant loss of lutein, along with formation of cis-lutein. Violaxanthin thermal degradation followed the

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epoxy-furanoid rearrangement, through two sequential transformations of 5, 6-epoxy to 5, 8-furanoid end-groups, giving rise respectively to luteoxanthin and auroxanthin. These facts indicated that isomerization and oxidation to both colored and non-colored compounds were the main reactions occurring during heating of carotenoids in aqueous-based and juice systems. It should also be noted that besides color fate, carotenoid oxidation compounds are also supposed to have detrimental effects in vivo, through the induction of oxidative stress, exertion of cytotoxic and genotoxic effects, and inhibition of gap junction intercellular communications (Caris-Veyrat, 2008). Polyphenolic compounds are also present in a range of exotic fruits i.e. protocatechuic, p-hydroxybenzoic, vanillic, syringic and ferulic acids, with vanillic and syringic acids, cyanidin, pelargonidin, peonidin and their glucosides (Fernandes et al., 2010). Pacheco-Palencia et al. (2009) demonstrated a detailed characterization of the polyphenolic compounds in the de-seeded fruits of Euterpe oleracea and Euterpe precatoria species. In their study, the overall thermal stability of polyphenolics in acai was evaluated by holding acai pulps at 80 °C for 1, 5, 10, 30, and 60 min, in the presence and absence of oxygen, as compared to a non-heated control. No significant differences (p b 0.05) were observed between the presence or absence of oxygen on polyphenolic degradation during heating. Extensive anthocyanin degradation occurred under these heating conditions, likely due to accelerated chalcone formation with prolonged anthocyanin exposure to high temperatures (Sadilova, Carle, & Stintzing, 2007). Anthocyanin degradation rates were directly related to thermal exposure times. Formation of cyclic-adducts might lead to a loss of an active OH-group in the meta-position of the A ring from the flavylium cation molecule mainly responsible for the loss in the antiradical activity in exotic fruits. It is quite well known that anthocyanin degradation is also

Fig. 2. Proposed mechanism for thermal degradation (90 °C) of all-trans-b-cryptoxanthin to colored compounds in aqueous-based systems. (Adapted from Zepka & Mercadante, 2009).

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reported as a result of indirect oxidation by phenolic quinones generated by PPO and peroxidase (Skrede, Wrolstad, & Durst, 2000). From the above results, it can be said that anthocyanin degradation was related to their structure, with o-diphenolic anthocyanins rapidly oxidized via coupled oxidation reactions and non-o-diphenolic anthocyanins slowly degraded by o-quinones or secondary oxidation products (Sarni-Manchado, Cheynier, & Moutounet, 1997). The first investigations on the degradation products of anthocyanins upon heating were carried out by Hrazdina (1971), Adams (1973), and Tanchev and Ioncheva (1976). Hrazdina (1971) reported that anthocyanin would decompose upon heating into a chalcone structure, the latter being further transformed into a coumarin glucoside derivative with a loss of the B-ring. Degradation is primarily caused by oxidation, cleavage of covalent bonds or enhanced oxidation reactions due to thermal processing. Thermal degradation of anthocyanins can result in a variety of species depending upon the severity and nature of heating. Fig. 3 shows the possible degradation pattern of anthocyanins in exotic fruits and formation of various intermediate compounds. It should be noted that the loss of bioactivity through anthocyanin degradation could not be compensated by the respective colorless phenolics generated upon heating (Sadilova, Carle, & Stintzing, 2007). Nevertheless, thermal processing may generate novel compounds not genuine to the particular commodity, thus enabling one to judge the heat burden the respective product has undergone. Understanding degradation mechanisms in exotic fruits is a prerequisite for maximizing bioactive compounds retention.

5.2. Non-thermal processing As outlined in Section 3 with the exception of ozone treatment most non-thermal processing methods result in better retention of bioactive compounds than thermal methods. Ozone treatments are expected to cause the loss of important phytochemicals, because of its strong oxidizing activity. In contrast, ultrasound treatments have

shown some promising results. Two main chemical reaction mechanisms have been proposed for sonication degradation. The first mechanism is pyrolysis within cavitation bubbles or nuclei; these are very small, free-floating bubbles in the liquid or gas pockets trapped in the crevices of the solid boundaries in the liquid medium (Sivasankar, Paunikar, & Moholkar, 2007), which is more likely to be the major reaction path for the degradation of polar bioactive compounds. Ascorbic acid degradation during sonication may be due to free radical formation (Portenlänger & Heusinger, 1992). Hydroxyl radical formation is found to increase with degassing. Sonication cavities can be filled with water vapor and gases dissolved in the liquid food, such as O2 and N2 (Korn, Prim, & deSousa, 2002). The interactions between free radicals and ascorbic acid may occur at the gas–liquid interfaces. In summary, ascorbic acid degradation may follow one or both of the following pathways: 1) Ascorbic acid → thermolysis (inside bubbles) and triggering of Maillard reaction. 2) The generation of •OH radicals (H2O → OH + •H) subsequently oxidizes the polar organic compounds (ascorbic acid, total phenols). Similarly degradation of lycopene might be due to the fact that the hydroxyl radicals produced by acoustic cavitation of ultrasound in extracts, with the presentation of a small amount of water, may result in the decomposition of lycopene, a lipophilic bioactive antioxidant. A possible mechanism of vitamin C degradation might be induction of electrochemical reactions by PEF treatments which might lead to changes of temperature, pH, and chemical composition (Loomis-Husselbee, Cullen, Irvine, & Dawson, 1991). For example, the release of Fe2+/Fe3+ from stainless steel electrodes (Loomis-Husselbee et al., 1991; Rajeshwar, Ibanez, & Swain, 1994) and the increase of oxygen dissolution might trigger oxidation of vitamin C in fluid foods. Not many authors have reported the effect of PEF treatments on individual anthocyanins in exotic fruits.

Fig. 3. Proposed mechanism for thermal degradation of anthocyanins. (Adapted from Sadilova et al., 2007).

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6. Strategies to improve retention during thermal and non-thermal processing Fruit products found in the market undergo commercial processing techniques most of which are thermal processing though some of the industries may have products processed by non thermal processing. In common with other bioactive compounds polyphenols, anthocyanins are enzymatically degraded in the presence of polyphenol oxidase. This enzyme can be inactivated by mild heating and therefore some authors have reported that the inclusion of a blanching step (heating to approximately 50 °C) can have a positive effect on anthocyanin retention. For example Skrede et al. (2000) demonstrated that addition of a blanched blueberry-pulp extract to blueberry juice resulted in no degradation of anthocyanins, whereas addition of an unblanched extract caused a 50% loss of anthocyanins, suggesting an enzymatic role in anthocyanin degradation. Rossi et al. (2003) suggested that an additional blanching step in juice processing may be vital, when evaluating fruit products for their health effects as blanching inactivates polyphenol oxidase. Similar approach can be opted for exotic fruits and their products. Higher stability of bioactive compounds can be achieved by process optimization. It is therefore assumed that the thermal burden during processing of bioactive compounds may further degrade them during storage. Intelligent selection of appropriate extrinsic storage condition systems based on detailed sequential studies is necessary to attain highest bioactive retention (Patras et al., 2010). The need to optimize processes in terms of bioactive retention in exotic fruits, demands more research to streamline processes by combinations of technologies, particularly with respect to optimization of practical applications. Process optimization of thermal processes in combination with non thermal technologies such as high pressure, ultrasound, pulsed electric field has significant potential for increased bioactive retention. 7. Conclusions and future trends Current knowledge indicates that in general high temperature treatments can affect levels of bioactive compounds in exotic fruits and their products. The mechanism by which bioactive compounds degrade are numerous, complex and perplexing, sometimes unknown. Ensuring food safety and at the same time meeting the demand for nutritious foods (bioactive compound retention), has resulted in increased interest in non thermal preservation techniques. The use of novel non-thermal processing is well known, several novel and interesting applications for improving the technological properties and the bioactivity of food have emerged during the past few years. These technologies represent a rapid, efficient and reliable alternative to improve the quality of food, but it also has the potential to develop new products with a unique functionality. However, in many cases the applicability of these technologies has not been investigated for exotic fruits and their products. Finally, it is worth noting that while many innovative food processing techniques have shown good potential for improving the nutritive quality of all processed fruits a significant proportion has not been scaled up for new food applications. A better understanding of the complex physicochemical mechanism of action of non-thermal processing technologies and their effects on the technological and functional properties of exotic fruits and their products, would also contribute to reinforce the presence of their applications. References Adams, J. B. (1973). Thermal degradation of anthocyanin with particular reference on glucosides of cyanidin. In acidified aqueous solution at 100 °C. Journal of the Science of Food and Agriculture, 24, 747−762.

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